Material screening includes the use of vibratory screening machines. Vibratory screening machines provide the capability to excite an installed screen such that materials placed upon the screen may be separated to a desired level. Oversized materials are separated from undersized materials. Over time, screens wear and require replacement. As such, screens are designed to be replaceable.
Replacement screen assemblies must be securely fastened to a vibratory screening machine and are subjected to large vibratory forces. Replacement screens may be attached to a vibratory screening machine by tensioning members, compression members or clamping members.
Replacement screen assemblies are typically made of metal or a thermoset polymer. The material and configuration of the replacement screens are specific to a screening application. For example, due to their relative durability and capacity for fine screening, metal screens are frequently used for wet applications in the oil and gas industry. Traditional thermoset polymer type screens (e.g., molded polyurethane screens), however, are not as durable and would likely not withstand the rough conditions of such wet applications and are frequently utilized in dry applications, such as applications in the mining industry.
Fabricating thermoset polymer type screens is relatively complicated, time consuming and prone to errors. Typical thermoset type polymer screens that are used with vibratory screening machines are fabricated by combining separate liquids (e.g., polyester, polyether and a curative) that chemically react and then allowing the mixture to cure over a period of time in a mold. When fabricating screens with fine openings, e.g., approximately 43 microns to approximately 100 microns, this process can be extremely difficult and time consuming. Indeed, to create fine openings in a screen, the channels in the molds that the liquid travels through have to be very small (e.g., on the order of 43 microns) and all too often the liquid does not reach all the cavities in the mold. As a result, complicated procedures are often implemented that require close attention to pressures and temperatures. Since a relatively large single screen (e.g., two feet by three feet or larger) is made in a mold, one flaw (e.g., a hole, i.e., a place where the liquid did not reach) will ruin the entire screen. Thermoset polymer screens are typically fabricated by molding an entire screen assembly structure as one large screening piece and the screen assembly may have openings ranging from approximately 43 microns to approximately 4000 microns in size. The screening surface of conventional thermoset polymer screens normally have a uniform flat configuration.
Thermoset polymer screens are relatively flexible and are often secured to a vibratory screening machine using tensioning members that pull the side edges of the thermoset polymer screen away from each other and secure a bottom surface of the thermoset polymer screen against a surface of a vibratory screening machine. To prevent deformation when being tensioned, thermoset polymer assemblies may be molded with aramid fibers that run in the tensioning direction (see, e.g., U.S. Pat. No. 4,819,809). If a compression force were applied to the side edges of the typical thermoset polymer screens it would buckle or crimp, thereby rendering the screening surface relatively ineffective.
In contrast to thermoset polymer screens, metal screens are rigid and may be compressed or tensioned onto a vibratory screening machine. Metal screen assemblies are often fabricated from multiple metal components. The manufacture of metal screen assemblies typically includes: fabricating a screening material, often three layers of a woven wire mesh; fabricating an apertured metal backing plate; and bonding the screening material to apertured metal backing plate. The layers of wire cloth may be finely woven with openings in the range of approximately 30 microns to approximately 4000 microns. The entire screening surface of conventional metal assemblies is normally a relatively uniform flat configuration or a relatively uniform corrugated configuration.
Critical to screening performance of screen assemblies (thermoset polymer assemblies and metal type assemblies) for vibratory screening machines are the size of the openings in the screening surface, structural stability and durability of the screening surface, structural stability of the entire unit, chemical properties of the components of the unit and ability of the unit to perform in various temperatures and environments. Drawbacks to conventional metal assemblies include lack of structural stability and durability of the screening surface formed by the woven wire mesh layers, blinding (plugging of screening openings by particles) of the screening surface, weight of the overall structure, time and cost associated with the fabrication or purchase of each of the component members, and assembly time and costs. Because wire cloth is often outsourced by screen manufacturers, and is frequently purchased from weavers or wholesalers, quality control can be extremely difficult and there are frequently problems with wire cloth. Flawed wire cloth may result in screen performance problems and constant monitoring and testing is required.
One of the biggest problems with conventional metal assemblies is blinding. A new metal screen may initially have a relatively large open screening area but over time, as the screen is exposed to particles, screening openings plug (i.e., blind) and the open screening area, and effectiveness of the screen itself, is reduced relatively quickly. For example, a 140 mesh screen assembly (having three layers of screen cloth) may have an initial open screening area of 20-24%. As the screen is used, however, the open screening area may be reduced by 50% or more.
Conventional metal screen assemblies also lose large amounts of open screening area because of their construction, which includes adhesives, backing plates, plastic sheets bonding layers of wire cloth together, etc.
Another major problem with conventional metal assemblies is screen life. Conventional metal assemblies don't typically fail because they get worn down but instead fail due to fatigue. That is, the wires of the woven wire cloth often actually break due to the up and down motion they are subject to during vibratory loading.
Drawbacks to conventional thermoset polymer screens also include lack of structural stability and durability. Additional drawbacks include inability to withstand compression type loading and inability to withstand high temperatures (e.g., typically a thermoset polymer type screen will begin to fail or experience performance problems at temperatures above 130° F., especially screens with fine openings, e.g., approximately 43 microns to approximately 100 microns). Further, as discussed above, fabrication is complicated, time consuming and prone to errors. Also, the molds used to fabricate thermoset polymer screens are expensive and any flaw or the slightest damage thereto will ruin the entire mold and require replacement, which may result in costly downtime in the manufacturing process.
Another drawback to both conventional metal and thermoset polymer screens is the limitation of screen surface configurations that are available. Existing screening surfaces are fabricated with relatively uniform opening sizes throughout and a relatively uniform surface configuration throughout, whether the screening surface is flat or undulating.
The conventional polymer type screens referenced in U.S. Provisional Application No. 61/652,039 (also referred to therein as traditional polymer screens, existing polymer screens, typical polymer screens or simply polymer screens) refer to the conventional thermoset polymer screens described in U.S. Provisional Patent Application Ser. No. 61/714,882 and the conventional thermoset polymer screens described herein (also referred to herein and in U.S. Provisional Patent Application Ser. No. 61/714,882 as traditional thermoset polymer screens, existing thermoset polymer screens, typical thermoset polymer screens or simply thermoset screens). Accordingly, the conventional polymer type screens referenced in U.S. Provisional Application No. 61/652,039 are the same conventional thermoset polymer screens referenced herein, and in U.S. Provisional Patent Application Ser. No. 61/714,882, and may be fabricated with extremely small screening openings (as described herein and in U.S. Provisional Patent Application Ser. No. 61/714,882) but have all the drawbacks (as described herein and in U.S. Provisional Patent Application Ser. No. 61/714,882) regarding conventional thermoset polymer screens, including lack of structural stability and durability, inability to withstand compression type loading, inability to withstand high temperatures and complicated, time consuming, error prone fabrication methods.
There is a need for versatile and improved screening members, screening assemblies, methods for fabricating screening members and assemblies and methods for screening materials for vibratory screening machines that incorporate the use of injection molded materials (e.g., thermoplastics) having improved mechanical and chemical properties.